The present invention relates to a protection system for pulverizer mills typically used in coal burning power plants, and other industrial coal burning facilities that may incorporate boilers, kilns or process heaters. An embodiment of the invention inhibits and suppresses fires, explosions and/or puffs in pulverizer mills, and provides control over high mill outlet temperature excursions. An embodiment of the invention can also be utilized on other solid fuel systems that incorporate pulverizer mills and/or pneumatic conveying systems for pulverized or granulated solid fuels. Various embodiments of the invention can include many advanced features and performance parameters providing benefits over existing steam, water, water fog or carbon dioxide inerting systems.
Fires and explosions in coal pulverizer mills can cause tremendous financial and operational burdens on coal fired power plants, as well as other coal fired boilers and industrial processes, especially those burning highly volatile coals such as Powder River Basin (PRB) coal. Along with posing a risk to worker safety, these events lead to financial losses incurred in repairs, lost power generation and litigation. Apart from regular mill maintenance, current techniques in dealing with fires and explosions rely on reaction to an event rather than prevention. Mill inerting, fire suppression, explosion venting and explosion suppression are all offline techniques, in that the mill must either be taken out of service for these techniques to be effective or the mill must be taken out of service following an explosion. Mill fires and explosions have many possible causes ranging from operator error to coal feed interruptions.
There are a variety of issues that can lead to mill fires or explosions. These may be maintenance related, caused by equipment failure or improperly following operational guidelines. However, many mill fires and explosions are caused by “hot restarts,” a standard operating procedure which is generally accepted in the industry. A hot restart refers to starting a mill immediately after a trip. Trips often occur while the pulverizer is loaded with coal. With vertical spindle style mills, a loss of airflow during such a trip means that coal that previously was suspended above the grinding bowl or table falls down to the hot underbowl area where temperatures often exceed 650° F. In this high temperature region of the mill, coal quickly dries and, especially in the case of PRB coal and similar highly volatile coals, spontaneously ignites and begins to smolder. When airflow is reintroduced, coal that was previously in mounds with little access to oxygen become suddenly suspended. Often these accumulations or mounds of coal are smoldering or burning because they have settled into a high temperature area of the coal mill, and are agitated and suspended in air. Once suspended, more surface area is exposed to oxygen, resulting in the often catastrophic combination of high air-to-fuel ratio, high temperatures and an ignition source that could result in an explosion.
Often mill fires start after a mill has stopped either due to a trip or as part of a controlled shutdown. Temperatures in the mill rise for a period after the mill is taken off when the heat stored in the thick metal mill housing migrates into the vessel. The resulting rise in temperature causes any coal remaining in the mill to dry and ignite. Left undetected, such fires can grow into major issues when primary airflow is reintroduced. Due to this threat, control room operators are prone to error. Operators are often tasked with watching indicated mill outlet temperature for hours after a mill is taken offline and introducing cold airflow if temperatures rise above normal operating levels.
While manual startup and shutdown is often preferred over automatic routines for a variety of reasons, a small oversight on the part of the operator may lead to catastrophic events. For instance, if a feeder is started late during the startup process, temperatures may spike because an absence of coal flow means an absence of the moisture content of the coal. If coal is introduced too late into the startup procedure for temperatures to be kept below blast gate trip temperatures, again the hazardous combination of high temperatures and dry coal is likely. During shutdown, if an operator fails to stop hot airflow when fuel feed is stopped, air-to-fuel ratio and temperature will go high, increasing the potential of an explosion or fire.
Mill fires have been known to erupt because a mill, still loaded with coal, which has been isolated from air supply, is opened for inspection. These fires most often occur when a mill has not fully cooled to ambient temperatures. Opening the mill stirs up previously settled coal dust and introduces O2 into a mill that may or may not have previously contained an inert atmosphere. This scenario may lead to injury or death.
Improperly maintained or otherwise malfunctioning equipment or measurement instrumentation is another major cause of mill fires. Coal feed interruptions, resulting from mechanical issues or plugged coal feed pipes, often result in high temperatures and air-to-fuel ratios. Improper airflow or temperature indications also have the potential of causing issues. For instance, an indicated temperature that is much lower than actual mill outlet temperature can lead to driving the mill temperature dangerously high. Improper airflow indication has the potential to lead to coal spillage into the underbowl because of insufficient airflow. Stuck or otherwise compromised hot or cold air dampers also have the potential of causing high temperatures, insufficient velocities or high air-to-fuel ratios, while worn or eroded pulverizer components may allow for coal to settle or spill over into the underbowl.
There are inerting systems and explosion suppression or venting systems known in the art. Inerting systems are designed to limit the amount of oxygen in the mill by injecting a noncombustible, nonreactive gas into the vessel. Gases used for this purpose are typically steam, nitrogen, carbon dioxide or flue gases. While inerting is effective at extinguishing smoldering or burning materials inside the mill, this method only works when a mill is isolated. This means that the mill operations, coal and airflow must cease and that all inlet and outlet gates are closed. Under these mandatory conditions the pulverizer is inerted and a based characterization test performed during commissioning. It is perceived that all other parameters that affected this characterization test remain constant after commissioning. However, if a damper leak is detected or an improper measurement of media flow rate, this may lead to oxygen levels that exceed the intended ten to fourteen percent oxygen by volume. Without reliable and continuous O2 measurement, such issues may go undetected. Steam inerting can be effective in displacing oxygen in the pulverizer, but it must be insured that the steam does not condense due to falling below saturation temperature and pressure. Generally, steam cannot be relied upon to extinguish a fire.
Explosion suppression and venting solutions react to dust explosions by either rapidly combining the combustible coal dust with a noncombustible dust (phlegmatization) and by venting the explosion pressure with explosion doors, respectively. These methods are purely reactive and result in pulverizer downtime and unit derates. In the case of explosion suppression systems, the mill must be isolated and cleaned out and the suppressant canisters reloaded. In the case of explosion venting, explosion doors or rupture disks or panels must be replaced. In either case, further downtime is incurred since the mill must be thoroughly inspected after an explosion.
Existing inerting systems concentrate on inhibiting mill fires during start up and shut down. Existing steam, water and carbon dioxide systems typically provide minimal or no fire suppression capability inside the pulverizer, while the mill is in service.
The objective of more rapid cooling of pulverizers is to shorten the time required for maintenance outages and inspections by permitting quicker entry into the pulverizer internals after it has been removed from service. If a fire occurs, the fire can be extinguished quickly preventing damage, costly repairs and extended downtime that is typical following a mill fire incident.
Temperatures within coal pulverizer mill internals vary greatly and can reach 700 degrees Fahrenheit during normal operation, especially while firing high moisture sub-bituminous coals. During normal and continuous operation, the highest temperatures are constrained or isolated to areas of the mills where there is usually no coal, dust or combustible material. Certain conditions such as interruptions in raw coal feed or other mechanical and operational anomalies can allow the high temperatures inside the mill to migrate to other areas of the pulverizer mill where pulverized and granulated combustion material (coal or other solid fuel) exist. This usually manifests itself as a temperature excursion where mill outlet or discharge temperature is abnormally high. There is a high risk of fires or puff evolving while mill outlet temperature is abnormally high.
Coal pulverizer mill fires and explosions present a major safety and financial concern for owners and operators of coal fired boilers and utilities. Such incidents can damage or completely destroy the mill and ancillary equipment. Workers in the vicinity of the mill may be injured or killed by thermal injury, hot gases and/or flying debris. Another concern is combustible dusts on and around ancillary equipment in the area that can result in secondary explosions or fires.
A commonly relied upon method of suppressing a fire inside a coal pulverizer/mill is increasing coal feed to flood the mill with fuel to decrease temperatures in the mill and smother the fire by inducing a fuel rich environment. Flooding the mill that is a closed system can reduce the air/oxygen available to support combustion inside the mill as well as maintain air to fuel ratios below the level necessary to support a pulverizer puff or explosion. Coal also contains moistures. Some coals are greater than twenty-five percent moisture by weight, and flooding the mill with moist fuel reduces temperature. To address internal mill fires, most fire suppression systems known in the art douse the mill externally with water, and are ineffective at suppressing fires inside the classifier and grinding/pulverization zones of coal mill/pulverizers. Other strategies to control, suppress or mitigate damage caused by a mill fire can include closing air inlet dampers (primary air dampers, hot air blast gates or other guillotine type isolation dampers) and fuel or burner lines or conduits using burner shut-off valves or isolation gates, in order to remove sources of oxygen (bottling), and filling the mill with steam or water fog (inerting). These methods typically require several hours to completely suppress a fire and most often do not suppress the fire quickly enough to prevent substantial damage to the mill or pulverizer system. Heat and combustibles, such as gases and coal dust remaining in the mill after suppression, present the risk of re-ignition. This system will address neutralizing the combustible material inside a mill while it is out of service as well as enhancing cooling limiting risk associated with re-ignition. The high temperatures inside the mill after suppression mean that a long cooling period is required before maintenance crews may enter the mill to assess and repair damage. Similarly, in non-emergency maintenance and inspection situations, the mill must be cooled from operating temperatures (it is also typical that the process of removing a pulverizer from service sometimes allow the mill to get heated above normal operating temperatures), adding several hours to the process of maintaining and inspecting the mill.
In addition, the numerous components typically contained in a coal pulverizer mill create a number of enclosed and isolated spaces within the coal pulverizer mill, in which fires can ignite. Accumulations and settling of fine coal particles inside the mill/pulverizer components can spontaneously ignite, particularly in mills with highly reactive sub-bituminous coals. In addition to spontaneously igniting, accumulations of coal pulverized to small particles can be easily ignited during the start-up process where these particles are agitated and exposed to a high air to fuel ratio environment as well as the possibility of high temperature excursions. Raw coal supply interruptions due to imprecise feeder control and stoppages above and below the feeder are another common source of fires and puffs. Interruptions in raw coal feed can be caused by environmental conditions such as frozen coal, wet coal from precipitation and mechanical anomalies such as broken feeder belts, seized bearings and other causes. Also, accumulations of raw coal that has spilled over into the under bowl section are exposed to temperatures of 500° F. to 750° F. while firing sub-bituminous coal, and are another common cause of mill fires. As such, there is a need for a fire suppression system that can effectively suppress and extinguish fires and explosions in all internal areas of the pulverizer mill.